The invention relates to a device for manufacturing profiled hollow tubing from synthetic plastics material, comprising an extruder with at least one annular mouth piece or opening, situated between a casing and at least one core, and two endless rows of consecutive die halves, each die half of which has a semi tubular profiled mould cavity, the rows being movable along endless guide ways extending parallel to each other in an operative track, the opposite halves of die in this operative track completing each other so as to form a hollow mould, comprising further a closing means retained in spaced relationship from the core and means for generating an inner overpressure in the space between the annular mouth piece and the aforementioned closing means, while each endless row consists of an inner and an outer half row, and spaced sprockets are disposed to move a die half from an outer half of row to an inner half of row in order to arrange two opposite die halves in such a way that they form a hollow mould.
Such a device is known per se. These known devices have the drawback that the halves of die complete each other with a tilting movement to a part of a hollow mould. During this tilting movement the velocity of the front- and back surfaces of the halves of die is greater at the moment of closing than the velocity in the preceding part of the track. This involves that the front faces of the die halves at the moment of closing of two opposite halves of die move faster than the soft polyvinyl chloride tubing which should be enclosed. As a consequence there is a possibility that the soft wall of the tubing is gripped by the halves of die and pushed forward whereby synthetic material lands in between the die halves. This synthetic material forms projections on the outer side of the tubing. The strength of the tubing is not helped by these projections and in general even a weakening is produced.
A greater drawback consists, however, in that such projections may cause grave disturbance in subsequent perforating apparatus.
Consequently much interest is taken in apparatus in which these difficulties are not experienced and in which the risk of the formation of projections owing to deformation of plastics between consecutive halves of dies in each endless row of die halves is avoided.